Knocking in Heater / AC Duct

I have a 94 Silverado, regular cab, with ac. I have just developed a rhythmic knocking in the heater / ac duct when the vehicle is started and shut off. I got down there and felt around while it was doing this and found a duct door on the far passenger side toward the bottom of the duct box and could feel it in this door. Any thoughts? Were there any service bulletins on this that anyone knows of? Thanks.

I have a 94 Silverado, regular cab, with ac. I have just developed a rhythmic knocking in the heater / ac duct when the vehicle is started and shut off. I got down there and felt around while it was doing this and found a duct door on the far passenger side toward the bottom of the duct box and could feel it in this door. Any thoughts? Were there any service bulletins on this that anyone knows of? Thanks.

Jim

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on my sons 91 it does this it is a clicking for about ten seconds on both start up and shuting engine off , i have fugured it is the "blend acuator" if i did no kill that word he has two it is the one on the right side the hardest to get at. hope this helps

OK. I found some other information which confirmed that it is the cold air blend actuator motor. A common problem, I am finding. I was a professional mechanic in the 80's through 95 and never had to deal with this back then. I will take pics and let everyone know how it goes. Thanks for the help.